This invention relates to a method for manufacturing a fused cast refractory such as an alumina-zirconia-silica series, alumina series or alumina-chromia series and a mold used for manufacturing the same.
In general, alumina-zirconia-silica electrofused cast refractories are referred to as AZS refractories and are widely used in the glass industry. AZS refractories are made through the steps of fusing or melting a blended raw material of plural composition in an arc furnace, casting the fused or melted material in a mold formed by silica sand and a binder for the sand, and thereafter gradually cooling (annealing) the cast product in alumina powder.
In order to form the mold, water glass (sodium silicate) is sometimes used as the binder for the silica sand. However, the water glass causes such drawbacks as sintering of the silica sand with the cast product, insufficient mold strength or strength deterioration due to absorption of moisture from the air, and formation of scattered small pores in the cast product by penetration of absorbed moisture into the cast product. Further, when water glass is used as the binder, the silica sand used here is difficult to regenerate for reuse, because it is exposed to a high temperature of about 1900.degree. C. for a long time during casting and annealing. Also, if once used, the silica sand shows reduced refractory properties due to the alkali residue from the water glass. Therefore, the silica sand cannot be reused.
An organic binder such as phenol resin or furan resin exhibits high binding strength with little deterioration over prolonged periods. However, the organic binders have effects detrimental to the product quality such as carbon diffusion from the organic binder onto the surface of the cast product, penetration of the gas generated from the organic binder into the cast product, and consequential formation of scattered small pores. Particularly recently the quality of refractories for use in a glassmelting furnace has become extremely important for the manufacture of high quality glass, for example, electronic glass such as cathode ray tubes and photomasks. If the refractories cause formation of bubbles in the glass at the glass/refractory interface of reaction, the glass cannot be used for electronic purpose.
Many theories and empirical rules have been proposed concerning the causes of bubble formation in molten glass. These theories teach the following requirements of the refractory for use in a glass-melting furnace:
(1) The refractory should have a non-porous dense structure. PA1 (2) The refractory should contain no iron or other metals, and also very little oxide of such metals. PA1 (3) The refractory compositions should maintain a high degree of oxidation.
In these requirements, there is no established method for measuring the degree of oxidation. At present, the only means to meet the third requirement is to maintain the highest possible state of oxidation at each step of melting, casting and gradual cooling, or annealing.
With a view to preventing the adverse effects on the cast product quality which result from an organic binder, it has been proposed in Japanese Published patent application Ser. No. 58-179536 to eliminate the adverse effects by advance blending of an oxidizing agent with the mold mixture to oxidize carbon and thereby prevent the carbon from the binder from lowering the state of oxidation of the refractory composition by reduction. However, oxidizing agents are expensive, and the chemicals require extreme care in handling. Also, the use of the organic binder invites the problem of NO.sub.x exhaust.
On the other hand, in order to cast a molten material the composition of which requires casting temperatures exceeding 2000.degree. C., such as an alumina or alumina-chromia electrofused cast refractory, a graphite plate is used as a mold material of desired shape and size. The graphite mold is used because no other material has been found which can resist the above high temperatures and the extremely rapid rate temperature rise. However, the graphite mold is very expensive, and normally a mold thereof shows local oxidation wear after three or four times of use and must be reprocessed before the next use. Consequently, the cast of mold material is a very large proportion of the production cost of the electrofused cast product.
An alumina refractory shows less tendency to be reduced during casting in comparison with an alumina-zirconia-silica refractory. However, because the mold is made of graphite, the alumina refractory reacts with the graphite of the mold during casting and forms many undesirable pores immediately below the refractory surface. Thereby, after surface processing of the refractory, the pores are exposed. Thus, the cast product has a serious quality defect. In certain cases, the refractory cannot be used unless the surface portion is cut off to a depth of as much as 50 mm.
Furthermore, the cast product must be released from the graphite mold at the earliest stage possible and buried in an alumina powder for gradual cooling. Otherwise, the cast product would develop fractures. The whole operation must be carried out under extremely high temperatures and therefore is severe.